What is a Saros series?

Because 223 synodic months is not identical to 239 anomalistic months or 242 draconic months, The 18-year saros periods do not endlessly repeat. Each series begins with the Moon's shadow crossing Earth near the north or south pole, and subsequent events progress toward the other pole until the Moon's shadow misses Earth and the series ends. A full series from start to finish lasts about 1,300 years. The August 21, 2017 eclipse is part of Saros Series 145 that includes 77 eclipses of which the August eclipse is number 22. All eclipses in this series occur at the ascending node of the lunar orbit. The series began with the partial solar eclipse of January 4, 1639 visible at the North Pole. The series will end with the partial solar eclipse of April 17, 3009 visible from the South Pole. The length of this series is 1,370 years. By the way, the total solar eclipse of August 21 is preceded two weeks earlier by a partial lunar eclipse on  August 7, 2017, which occurs during the same eclipse season when the sun is nearest this node.